The present invention relates to a motion system comprising an arrangement of rigid stages, flexure constraint modules, actuators, and sensors. This unique arrangement results in large motion range along with high motion quality, while using standard and commonly available components. Motion quality, in the context of a motion system, is defined in terms of precision, also known as bi-directional repeatability of motion; accuracy, also known as trueness of motion; and, resolution, also known as minimum incremental motion.
In the relevant art, a ‘motion system’ is understood to be a system that enables the motion of a rigid body or stage, commonly referred to as the Motion Stage, in a controlled fashion so as to follow a desired motion trajectory with respect to a reference Ground stage. In particular, the motion system does not refer to a specific component such as the bearing that guides the motion, or the actuator that generates the motion, or the driver that operates the actuator, or the sensor that measures the motion, or the electronics that is used with the sensor, or the controller that controls the motion. Instead, a motion system is a combination of one or more these components. The term ‘motion system’ is used here in the context of this generally accepted definition.
The directions along which a motion system provides motions or displacements at the Motion Stage are referred to as the ‘degrees of freedom’ (DoF), which can be translational or rotational. A motion system that provides displacement at the Motion Stage along a single direction is referred to as a single-DoF motion system; likewise, a motion system that provides displacements at the Motion Stage along multiple directions is referred to as a multi-DoF motion system. A motion system may provide a maximum of six DoF at the Motion Stage—three translations (typically along the X, Y and Z directions), and three rotations (about the X, Y and Z directions).
Motion systems that are capable of nanometric or sub-nanometric motion quality in terms of precision, accuracy, and resolution are also referred to as ‘nanopositioning systems’ in the relevant art. This present invention more specifically relates to multi-DoF nanopositioning systems capable of large motion range along each DoF direction, while using commonly available components.
Existing multi-DoF motion systems that provide nanometric motion quality are limited to hundreds of microns in motion range. Compact, desktop-size, and multi-DoF motion systems that can provide motion range of the order of several millimeters and yet achieve nanometric motion quality are desirable in a broad range of applications including scanning probe microscopy, nanolithography, single molecule experiments, molecular spectroscopy, drug discovery applications, hard-drive testing, micro and nano manipulation, and bio-imaging for stem cell research, to name a few.
There have existed several challenges in achieving the large motion range and high motion quality simultaneously in multi-DoF motion systems. One of the most fundamental of these challenges is the choice and design of a motion bearing that provides guided motion along multiple DoF directions. Several existing motion bearing methods are described next.
Motion guidance and load bearing in magnetic bearings is achieved by means of an advanced magnetic circuit design that is stabilized by feedback controls. With single-DoF magnetic bearing based systems, one can achieve large motion range as well as very high resolution, owing to non-contact operation. Since the motion bearing is a challenging sub-system in itself, the resulting motion systems are typically characterized by high complexity, cost, and maintenance, and relatively large sizes. Magnetic bearings are primarily suited for single-DoF motion systems. Multi-DoF system may be created by serially stacking multiple single-DoF motion systems, one on another.
Air bearings are also capable of large range and very high resolution due to the lack of physical contact between moving parts, but are suited for single-DoF motion systems, as in the previous case. An air bearings based multi-DoF motion system may be produced by serially stacking single-DoF systems. Such serial designs are generally bulky and involve moving cables and actuators, which pose a challenge for high precision, speed-of-response, and ease of assembly. Furthermore, air bearings need a constant supply of clean, high-pressure and low-humidity air, require periodic filter changes, and are not suitable for vacuum environment.
The traditional bearing technology for motion systems employs either rolling joints (e.g. ball bearings) or sliding joints (e.g. guiderails). Multi-DoF systems may be constructed via either serial or parallel kinematics. Precision ground, highly accurate, and pre-loaded recirculating ball bearings may be used to provide motion guidance; precision micrometers, lead-screws, or ball-screws may be used to transmit the motion from the actuator to the bearing stage. Despite utmost care in manufacturing and assembly, it is extremely difficult to improve the motion quality beyond 100 nm in these systems due to non-deterministic effects such as rolling of balls, sliding of surfaces, interface tribology, friction, and backlash.
Another alternative in bearing design—the coarse-fine scheme—has also been used to achieve the large motion range and high motion quality objective by mounting a small-range high-quality fine flexure stage, described below, on a traditional large-range lower-quality coarse stage. This arrangement results in additional complexity in terms of parts, assembly and operation, and is still not able to achieve high precision or bi-directional repeatability.
Flexure bearings are the most common and practical bearing choice for desktop-size nanopositioning systems. A monolithic construction entirely eliminates friction and backlash allowing theoretically infinite resolution and repeatability. Monolithic construction also reduces part counts and assembly steps, requires zero maintenance, provides infinite life when designed properly, and can operate in any kind of vacuum or harsh environment. Multi-DoF motion systems based on flexure bearings may be constructed via either serial-kinematics or parallel-kinematics.
A serial-kinematic multi-DoF motion system comprises of multiple single-DoF motion systems stacked one on another serially. However, this configuration is often bulky, and results in moving cables and actuators. Moving cables are a source of disturbance and affect the motion quality, while moving actuators represent large moving masses that are detrimental to the dynamic performance of the motion system. Furthermore, moving connections and actuators are difficult to implement in micro-scale applications, for example Micro Electro-Mechanical Systems (MEMS). Parallel-kinematic designs are free of these problems because they employ ground-mounted actuators and are often more compact and economical. Compared to serial-kinematic designs, the main drawbacks of traditional parallel-kinematic designs include relatively smaller motion range, potential for over-constraint, and greater error motions. Furthermore, parallel kinematic designs are not obvious and therefore are not as straightforward to design as serial kinematic designs. Despite all these factors, parallel kinematic designs generally more preferable due to their compactness, motion quality and manufacturability.
The design of a large range parallel kinematic multi-DoF flexure bearing is non-obvious and challenging. Furthermore, to achieve large motion range and high motion quality simultaneously in a motion system, the selection of practically feasible and commonly available actuators and sensors, and their integration with the flexure bearing are equally important. In general, this is a challenge because commonly available actuators and sensors have several limitations that restrict their use in multi-DoF nanopositioning systems.
It is important that the motion system design be such that commonly available actuators may be used while exploiting their specific advantages and accommodating their limitations. As stated earlier, a parallel kinematic configuration is preferable which means that all the actuators should be ground mounted, i.e. their Stators are fixed with respect to the reference Ground of the motion system. For effective ground-mounting, the attribute of ‘actuator isolation’ is important in a motion system. In a multi-DoF motion system, actuator isolation implies that the actuation for one DoF does not produce any displacements at the point of actuation for any other DoF; furthermore, the point or location on the flexure bearing along which actuation for a particular DoF is applied, should move along the direction of the applied actuation only and not otherwise. It should be obvious that good actuator isolation allows easy ground mounting of actuators, which is important in parallel-kinematic designs. More importantly, actuator isolation enables the use of commonly available linear actuators.
In a general, an actuator comprises a Stator and a Mover. In a motion system, the Stator is attached to one rigid body and the Mover is attached a second rigid body. The actuator produces an actuation force or displacement between the Stator and Mover, and this actuation is transmitted between the two associated rigid bodies. Commonly, the Stator is attached to a static Ground stage, while the Mover is attached to a moving stage. However, this arrangement may be reversed depending on the design, configuration, and assembly of the motion system. In some instances, neither of the rigid bodies involved is a static Ground stage.
While commonly available linear actuators can provide large motion range, or high motion quality, or both, they provide this motion along their own well-defined ‘actuation axis’, which has to be lined up with the appropriate point of actuation on the flexure bearing. These actuators typically do not tolerate any deviation from their actuation axis. If a flexure bearing is such that the point of actuation for a certain DoF drifts off from the actuator's actuation axis, then upon assembly the motion system will very likely suffer from binding, ultimately leading to damage of the flexure bearing and/or the actuator. Thus, actuator isolation is critical in a motion system to achieve large stroke and high motion quality using common actuators.
Some specific examples of actuators are provided here to highlight the above described limitation of common actuators. Piezo-electric actuators, typically based on Lead Zirconate Tintanate (PZT) ceramic stacks provide extremely high motion resolution, although their motion range is small. However, any loads acting in directions other than the axis of the brittle ceramic stack, which is also the actuation axis, cause permanent damage to the actuator. ‘Inch-Worm’ style actuators, based on a repetitive hold-step-release action achieved by means of an array of piezo-electric ceramics, provide large motion range and high motion resolution. But here also this motion is strictly guided along a specified axis. Electromagnetic actuators, such as voice-coils, provide large range and high resolution, but also have to be guided along the coil's axis, which becomes the actuation axis, to ensure uniform and useful actuation-force generation. Electrostatic actuators are an example of actuators that do not have to be guided along a specified axis and are relatively insensitive to off-axis displacements. However, they provide relatively lower force capability, and therefore are impractical for many motion systems.
Moreover, actuator isolation in a motion system also eliminates the need for a dedicated bearing for the actuator and a decoupler between the actuator's Mover and the point of actuation on the flexure bearing. Since the flexure bearing provides guided motion at the point of actuation, the Mover of the actuator may be directly connected to this location on the flexure bearing. This reduces overall size, number of parts, and complexity in the design.
The sensing demands for large range, high motion quality, and multi-DoF motion systems are equally challenging. Given the high motion quality requirement, end-point measurement of the displacements along the DoF is essential. End-point measurement implies an absolute measurement of the displacements of the Motion Stage along the DoF directions, with respect to Ground. In addition, multi-DoF measurements demand that the sensor for one DoF be tolerant of displacements along the other DoF. While Linear Variable Differential Transducers (LVDT) and linear optical encoders provide large measurement range and high measurement resolution, they have a well-defined axis of measurement, also known as the sensing axis. These sensors are restricted to measurements along the sensing axis and are intolerant to any motion that deviates from the sensing axis. Capacitance probes, on the other hand, provide very high resolution and tolerate large off-axis displacements, making them highly suitable for multi-DoF motion system. However, with nanometric resolution, their measurement range is typically limited to hundreds of microns, and therefore do not readily meet the desired objective of large motion range and high motion quality. Similarly, strain gauges and piezo-resistive sensors can provide nanometric resolution but at the cost of measurement range; moreover, they are also limited in terms of measurement accuracy. Laser interferometry is one of the few sensing options that provide large range, high resolution and tolerance to off-axis displacements. Yet, it is an impractical option for desktop-size nanopositioning systems, given the associated equipment size, lack of compact packaging, and high cost.
Because of these limitations in flexure bearings, actuators, and sensors, multi-DoF motion system designs that provide large motion range and high motion quality are not found in the prior art.